THE FAR INFRARED SPECTRUM OF $H_{2}O_{2}$. FIRST OBSERVATION OF THE STAGGERING OF THE LEVELS

High resolution spectra of $H_{2}O_{2}$, recorded by means of Fourier transform spectroscopy between 30 and $460 cm^{-1}$ have been analysed leading to the determination of the rotational levels of the torsional states $(n, \\tau)$ for $n=0,1,2,3$. To reproduce these energy levels. Watson type Hamiltonians have been used and it has been possible to observe the staggering of the levels due to the cis-barrier for n=2 and 3. The torsional hand centers have then been fitted using a torsional Hamiltonian of the form $\\left\\{B_{\\gamma\\gamma},J^{2}_{\\gamma}\\right\\} + V(\\gamma)$ with the potential energy function $V(\\gamma)$ written as $V(\\gamma) = V_{1} \\cos 2\\gamma + V_{2} \\cos 4\\gamma + V_{3} \\cos 6\\gamma + V_{4} \\cos 8\\gamma$ where the torsional coordinate $2\\gamma$ is the dihedral angle defining the relative position of the two O-H honds. The potential function constants obtained are $\\begin{array}{ll}V_{1} = 1036.97 \\pm 23.1 cm^{-1}, &V_{2} =657.53 \\pm 5.2 cm^{-1}.\\\\ V_{3} = 50.89 \\pm 3.3 cm^{-1} , V_{4} = 2.524 \\pm 0.83 cm^{-1}\\end{array}$$ corresponding to the barrier heights $V_{trans} = 387.07 \\pm 0.20 cm^{-1}\\quad V_{CIs} = 2563 \\pm 70 cm^{-1}$$ and to potential energy minima located at $2\\gamma = 111.9. \\pm 0.4$ from the cis-configuration. It is also shown that the rotational constants derived through the fit of the experimental rotational levels cannot be reproduced using a model which does not take into account the vibrational corrections.